Electrodeposition and characterization of Pt and PtCu coatings for methanol oxidation in alkaline medium / Siti Norsafurah Ab Malek

In this research, Pt based coatings (i.e.: pure Pt and PtCu alloys) were successfully electrode posited on the Ti substrate using chronoamperometry to investigate the methanol oxidation reaction (MOR) in alkaline medium. The main problems dealing with the Pt bulk electrode surface during methanol oxidation are: i) low surface area of the pure Pt bulk leading to low current density for methanol oxidation reaction and ii) the formation of intermediate adsorbed CO species during methanol oxidation reaction which inhibits the catalytic function of Pt. Therefore, the aim of this research is to prepare high surface area Pt based coatings with good catalytic performance and high stability which can offer an alternative pathway for methanol oxidation reaction. Deposition parameters such as deposition potential, time and molar ratio of electrolytes were optimized in order to produce the best Pt based coatings. FESEM analysis showed that different applied potential used during deposition process has produced different morphologies of Pt coatings such as cauliflower-like, durian-like, needle-like and worm-like Pt structures which are due to different nucleation rate and growth rate during deposition process. The findings revealed that the best pure Pt coating with durian-like structure was prepared via chronoamperometry by applying a constant potential of -0.20 V for 60 min from 0.3 M H2SO4 containing 0.01 M H2PtCl6.6H2O. The coating labelled as Pt(-0.20 V, 60 min) produced the highest ECSA (i.e.: 8.08 m2 g-1), current density peak (i.e.: 170.38 mA cm-2), If/Ib ratio (i.e.: 2.63) and the lowest onset potential (i.e.: -0.50 V) for methanol oxidation reaction in alkaline medium as compared to other pure Pt coatings. Subsequently, the addition of second metal (Cu) to pure Pt was found to give positive impact by minimizing the CO poisoning effect and reducing the Pt loadings on Ti substrate with excellent catalytic activity for methanol oxidation reaction. The best PtCu alloy coating with 81% Pt and 19% Cu (labelled as Pt81Cu19(-0.30 V, 30min)) was prepared by chronoamperometry at -0.30 V for 30 min showing the most significant enhancement of catalytic performance of Pt for MOR (ECSA: 12.07 m2 g-1, current density peak: 196.32 mA cm-2, If/Ib ratio: 3.60 and onset potential: -0.54 V) as compared to other PtCu alloys composition. EDX and ICP-OES analysis showed that the content in PtCu alloy increased with increasing Cu concentration present in the Pt:Cu electrolyte solution. XRD confirmed that Cu was inserted into a face-centered cubic Pt structure forming PtCu alloys. Meanwhile, XPS analysis showed that the Pt peak for PtCu alloy shifted to lower binding energy than that of pure Pt, indicating the electron transfer occurred from Cu to Pt due to its electronegativity differences. The shift in electron transfer from Cu to Pt would increase the valence electron (5d) vacancy of Pt, thus contributing to the modification in the electronic properties of Pt at Pt81Cu19 alloy, hence weakening the Pt-CO bonding energy. EIS analysis revealed lower electron transfer resistance of the Pt81Cu19 alloy surface coating/electrolyte interface indicating faster electron transfer kinetics on its surface as compared to pure Pt, thus, enhancing MOR. In addition, the Pt81Cu19 alloy coating showed slower current degradation corresponding to better stability with higher resistance to CO poisoning effect during MOR than pure Pt. These findings contribute to the basis and further understanding about the relationship between the deposition parameters towards surface properties and electrocatalytic behaviour of the prepared Pt based coatings for methanol oxidation reaction in alkaline medium.